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arXiv 2007.11159math.KT
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离散赋值环的$\mathrm{SL}_2$的同调

The homology of $\mathrm{SL}_2$ of discrete valuation rings

Kevin Hutchinson, Behrooz Mirzaii, Fatemeh Yeganeh Mokari

摘要:

设$A$是一个离散赋值环,它具有分数域$F$和(足够大的)剩余域$k$。证明了存在自然正合序列$H_3(\mathrm{SL}_2(A),\mathbb{Z}[\frac{1}{2}]) \to H_3(\mathrm{SL}_2(F),\mathbb{Z}[\frac{1}{2}])\to \mathcal{RP}_1(k)[\frac{1}{2}]\to 0$,其中$\mathcal{RP}_1(k)$是$k$的加细剪同余群.令$Γ_0(\mathfrak{m}_A)$表示$\mathrm{SL}_2(A)$中的矩阵组成的同余子群,其下非对角项位于极大理想$\mathfrak{m}_A$中。证明了存在正合序列$0\to \overline{\mathcal{P}}(k)[\frac{1}{2}]\to H_2(Γ_0(\mathfrak{m}_A),\mathbb{Z}[\frac{1}{2}])\to H_2(\mathrm{SL}_2(A),\mathbb{Z}[\frac{1}{2}])\to I^2(k)[\frac{1}{2}]\to 0$,其中$I^2(k)$是Grothendieck-Witt环$\mathrm{GW}(k)$的基本理想的二次幂,$\overline{\mathcal{P}}(k)$是$k$的剪同余群(在Dupont-Sah意义下)$\mathcal{P}(k)$的某个商.

英文摘要:

Let $A$ be a discrete valuation ring with field of fractions $F$ and (sufficiently large) residue field $k$. We prove that there is a natural exact sequence $H_3(\mathrm{SL}_2(A),\mathbb{Z}[\frac{1}{2}]) \to H_3(\mathrm{SL}_2(F),\mathbb{Z}[\frac{1}{2}])\to \mathcal{RP}_1(k)[\frac{1}{2}]\to 0$, where $\mathcal{RP}_1(k)$ is the refined scissors congruence group of $k$. Let $Γ_0(\mathfrak{m}_A)$ denote the congruence subgroup consisting of matrices in $\mathrm{SL}_2(A)$ whose lower off-diagonal entry lies in the maximal ideal $\mathfrak{m}_A$. We also prove that there is an exact sequence $0\to \overline{\mathcal{P}}(k)[\frac{1}{2}]\to H_2(Γ_0(\mathfrak{m}_A),\mathbb{Z}[\frac{1}{2}])\to H_2(\mathrm{SL}_2(A),\mathbb{Z}[\frac{1}{2}])\to I^2(k)[\frac{1}{2}]\to 0$, where $I^2(k)$ is the second power of the fundamental ideal of the Grothendieck-Witt ring $\mathrm{GW}(k)$ and $\overline{\mathcal{P}}(k)$ is a certain quotient of the scissors congruence group (in the sense of Dupont-Sah) $\mathcal{P}(k)$ of $k$.

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